My invention relates to a valve for regulating the delivery of air pressure to brakes as a component of an electronically controlled brake system. Most known pneumatic systems incorporating electronic controls allow the brakes to be operated normally until a skidding condition is detected at one of a vehicle's wheels. Once wheel skidding is detected, normal braking operations are overridden and air pressure is released from the appropriate brake until the condition of wheel skid is no longer detected. Such pneumatic systems are known to experience a variety of problems including delayed response time and overshooting or undershooting optimum braking performance.
A typical brake valve modified for use in an anti-skid system is disclosed in U.S. Pat. No. 3,768,519 to Morris. Under normal braking conditions, the brake valve operates as a three-way relay valve controlled by pilot pressure from an operator-controlled foot pedal. When pressure is applied to the foot pedal, pressurized air enters a control chamber and moves a pilot piston closing off a brake exhaust passage and releasing pressurized air into a delivery chamber for the brakes. Pressure increases in the delivery chamber opposing the pilot piston until an approximate equilibrium is reached with the control chamber which closes off the further supply of pressurized air to the delivery chamber. When the pilot pressure in the control chamber is decreased, pressurized air is released from the delivery chamber until a new equilibrium is reached.
The valve of U.S. Pat. No. 3,768,519 also incorporates electronic controls to further regulate pilot pressure in response to a signal indicating a wheel skid condition. When a wheel skid condition is detected, a solenoid valve opens an exhaust passage from the control chamber and blocks further communication between the foot pedal and the control chamber. The release of pilot pressure causes the relay valve to exhaust a proportional amount of pressure from the brake delivery chamber. Once the skid signal is terminated, the exhaust passage is closed and the brake pedal is placed back in communication with the control chamber. Thus, the known solenoid-operated valve provides for overriding the normal operation of a brake relay valve to temporarily relieve excess pressure applied to the brakes.
Although such valves provide some improvement in braking performance, they tend to react quite slowly to dynamic changes in braking conditions. For example, the rate at which brake pressure can be decreased is delayed by the time it takes to release required quantities of air from the control chamber. Similarly, the rate at which an increased amount of pressure can be added to the brakes is delayed by the time it takes to fill the control chamber with the required amount of air. Further, there are delays associated with sequentially opening and closing the required passages for controlling pilot pressure as well as delays associated with opening and closing passages in response to the pilot pressure.
U.S. Pat. No. 3,857,615 to Acar proposes to vary the rate at which air pressure is released from the control chamber to accommodate different braking conditions. For example, when a large amount of pilot pressure is applied in the control chamber, a much faster release of pressure is allowed than when pilot pressure is somewhat smaller. This feature helps to limit the amount brake pressure overshoots or undershoots a desired braking value in a slowly changing braking environment.
However, the amount of braking pressure applied remains time dependent. That is, the instantaneous pressure in the delivery chamber remains a function of the length of time at which the control valves are opened and closed. Further, the time required to change pressure levels in the delivery chamber depends on the volume of air that must be received or discharged from the control chamber. In an actual braking environment where braking conditions change dynamically, braking response of the known valves may lag so far behind changes in the braking environment that the known brake valves may be continuing to discharge pressure when increased brake pressure is required and vice versa. Thus, the known valves provide for a very inefficient braking response which fail to make use of all of the available traction.
In trucks, the brake valves associated with different wheels are positioned at significantly different distances from the operator-controlled foot pedal. During normal braking operations, the response time of the rear brakes may lag behind the response of the front brakes. Timing valves are known which delay the response of the front brakes to at least approximately match the response time of the rear brakes, but this also delays an initial braking response.